In the foundry art, cores or molds for making metal castings are normally prepared from a mixture of an aggregate material, such as sand, and a binding amount of a binder or binder system. Typically, after the aggregate material and binder have been mixed, the resulting mixture is rammed, blown or otherwise formed to the desired shape or pattern and then cured with the use of catalysts and/or heat to a solid, cured state.
A variety of different processes for forming molds and cores have been developed in the foundry industry. One type of process known as the shell molding process, is well known in the art. While there are many variations of this process, the process essentially comprises depositing a combination of sand and potentially thermosetting resin against a heated pattern such that the resin melts and cures to form a rigid shell mold or core section for use in the casting of metals. The combination of resin and sand used in the process can be a mixture of powdered resin and sand, or a free-flowing coated sand in which each grain is coated with a nontacky layer of resin.
The production of a core or mold by the shell process involves two basic steps, the invest and the cure step. In the first step, the resin-coated sand is dumped onto or blown against the heated metal pattern. The resin-coated sand is held against the pattern (invested) until the shell is thick enough to hold metal in a given application. In the second step, the resin-coated sand is dumped or dropped away from the shell of bonded coated particles of sand and the resulting shell is cured. After the shell is cured, it is removed from the hot metal pattern and is ready for use.
Another process, known to the art as the "no-bake" process, is also used in forming resin cores. This process requires no external heating. Instead, curing is accomplished by means of a catalyst added just before the sand and resin components are introduced into the core box. Base-cured resin components used in the no-bake process are generally mixtures of polyols and polyisocyanates. Solutions of these components are usually coated on the sand immediately before use.
A third process for making cores and molds employs sands treated with core oil mixes. These mixes contain drying oils and cereal binders. Cores and molds made with such core oil mixes are cured by baking them in an oven.
In all of these processes, the binder which has been mixed with sand acts, when cured, to bind the particles of sand in the form of the pattern. The core or mold must be strong enough to contain the molten metal until it solidifies. For this reason, a core or mold with high tensile strength is required.
One factor influencing the tensile strength of the cores and molds is the quality of the sand used in their preparation. When a silica sand is employed, it is generally necessary to use a sand or high purity. In the past, when silica sands of lower purity were used, it was necessary to add large amounts of binder to ensure structural integrity of the mold. This was not only costly but led to other undesirable results when gaseous decomposition products of the excess resin penetrated into the molten or solidifying metal resulting in pinholes and scarring of the metal shape.
Impure silica sands, such as lake and bank sands, are readily available in many areas of the United States. These impure sands are sometimes beneficiated by various processes such as water washing. However, it is still necessary to use excess binder with the washed sands to obtain the desired tensile strength of the cores and molds made from them. It is therefore desirable to develop a process whereby these inexpensive sands can be used to make foundary cores and molds without the need to use excess binder with the sand.
Bushey described a method for treating zircon-containing sands, U.S. Pat. No. 4,115,345, and olivine sands, U.S. Pat. No. 4,154,894, with an alkali metal silicate to improve the tensile strengths of resin shell molds or cores made from the sands. However, he reported that when this method was used with silica and chromite sands, no improvement in the tensile strength of the cores and molds was observed.
A process has now been discovered which permits the use of impure silica sands in conjunction with moderate amounts of binder to form foundry cores and molds with improved tensile strength. This process is less expensive than present beneficiation methods and gives cores and molds with improved tensile strengths.
A further unexpected benefit of using these treated sands is that cores prepared from them by the base-curing "no-bake" process are more readily released from the core box. Easy release of the cores is commercially important, since sticking cores slow down the core-making process and often become broken and useless.